ETS-Lindgren 5411 GTEM! Test Cell - Manual (Page 59)

Manual EUT Manipulator Operation Theory
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9.0 Manual EUT Manipulator Operation Theory
Measurements in a GTEM!
A Gigahertz Transverse Electromagnetic (GTEM!™) Cell may either receive or
transmit; thus electromagnetic interference (EMI) measurements in a
GTEM! may be of either emissions or immunity. Among emissions
measurements are far-field, near-field, and some special measurements. To
predict the performance of the Equipment Under Test (EUT) during
measurements of emissions on an Open Area Test Site (OATS), the emissions
must be measured in a specific set of positions in the GTEM!. Predictive EUT
performance on an OATS by making measurements in a GTEM! is also called
correlation, and the mathematical process is often called the correlation
algorithm.
F
AR
-F
IELD
M
EASUREMENTS
Simplified far-field measurements may be made of EUT in a GTEM! to predict
or correlate OATS-measured emissions. These measurements are usually made
to show compliance to standards, such as CISPR 22 or FCC Part 15, in which it
is only necessary to know the maximum E-field versus frequency within a
specified range of heights at a certain distance. For example, FCC tests for home
computers search heights from one to four meters above the ground at a
distance of three meters over the frequency range of 30 MHz to 5 GHZ. These
simplified measurements require emissions to be measured with the EUT in only
three orthogonal positions. This is called the three-measurement, three-input
correlation algorithm. The main simplifying assumption in this algorithm is that
the EUT has gain no greater than a dipole; for example, a dipole radiation
pattern.
N
EAR
-F
IELD
M
EASUREMENTS
Near-field measurements may be made of EUT in a GTEM! to correlate
emissions over the frequency range of 9 kHz to 30 MHz. This is called the
nine-measurement, nine-input correlation algorithm, and requires measurement
of emissions with the EUT in nine positions. The EUT is assumed to be much
smaller than a wavelength in the largest dimension, a reasonable assumption
below 30 MHz for EUT that will fit into a GTEM!. While this algorithm was
originally intended for near-field measurements below 30 MHz, it also works well
for far-field measurements above 30 MHz and should be valid from 9 kHz to
5 GHz.